Small electronic appliances are today used in a wide variety of applications and have become a ubiquitous part of modern society. These applications include computers, telephones, and home entertainment system components, among many others. One reason for the widespread use of these appliances is that recent advances in technology have expanded their capabilities while at the same time lowering their cost. A key part of this advancing technology has been the development of improved semiconductor devices and the processes for making them.
Semiconductors are materials that after being properly treated conduct electricity when placed under certain conditions, which often include the presence of a small electrical charge. This enables the manufacture of solid-state switches—those that have no moving parts. Other standard (and new) electrical devices can be created out of semiconductors as well. In addition to having no moving components parts that are subject to fatigue or other mechanical failure, solid-state devices can be fabricated in extremely small sizes. Very small, even microscopic electrical components are now used to provide the multitude of resistors, switches, and capacitors necessary for today's electronics applications.
The processes used to fabricate these tiny semiconductor devices are numerous, but the basic process may be described generally. A material, such as silicon, is produced for use as a base, or substrate, upon which various electrical components will be built. This material is then formed into an appropriate shape, usually a thin slice called a wafer. The pure silicon is then selectively treated with one or more materials called dopants, such as ionized boron or phosphorus. The introduction of these impurities begins the process of creating the desired semiconductive properties. Various structures may then be formed at or near one surface of the wafer to construct the desired components.
An exemplary semiconductor device, in this case a transistor, is shown in FIG. 1. FIG. 1 is an illustration showing in cross-section the basic components of a MOSFET (metal-oxide semiconductor field effect transistor) 10. In this example, silicon forms the substrate 15 upon which various devices may be fabricated. The transistor includes a gate 20 having a gate electrode 25 the gate electrode made of a conductive material such as polycrystalline silicon (poly) or a metal. Separating the gate electrode from the substrate 15 is a thin gate dielectric layer 30. In the transistor 10 of FIG. 1, spacers 35 are positioned on either side of the gate electrode 25. Conductive regions called a source 40 and a drain 45 are formed in the substrate 15 on either side of the spacers 35. Source 40, drain 45, and gate electrode 25 are each coupled, respectively, to electrical contacts 50, 51, 52, each of which may in turn be connected to external components (not shown) so that electrical current may flow to and from these transistor components when appropriate. When a small electrical charge is applied to gate electrode 25 via contact 52, then current may flow between drain 45 and source 40 via channel region 5.
All of the transistors in a particular application need not be, and usually are not identical. For example, a MOSFET having a source region and drain region doped with boron ions creates a p-channel device (having positive charge carriers and activated by a negative gate voltage), often referred to as a PMOS device. An NMOS device, on the other hand, has a source region and drain region created with an n-type dopant such as phosphorous ions. These two types of MOSFETs may be use to advantage in pairs, creating a CMOS (complimentary-MOS) device. CMOS devises, combined with appropriate logic configurations, result in significant power savings and are currently widely used throughout the semiconductor industry.
As another example, MOSFETs may be fabricated to have either a higher or a lower threshold voltage, relative to each other. A multi-threshold device has both, and these may be used to advantage where application logic is available to choose the most appropriate type for a given operation. Multiple threshold voltage processes offer a way to reduce total power consumption while maintaining performance. A low transistor threshold voltage may be used on critical-path operations to meet timing constraints. Other operations may be assigned a higher threshold voltage to reduce the sub-threshold leakage component of static power consumption. Multiple threshold voltage transistor devices are becoming popular and are frequently used.
These MOSFET transistors are very small, for example, gate length (distance between the source and the drain) of MOSFET 10 may be no more than 100 nm in width. As gates become smaller, certain undesirable characteristics become more pronounced. Problems caused in this manner are sometimes labeled short-channel effects (SCE). In some cases, these SCE problems may be mitigated or avoided through the use of new types of materials. The gate electrode, for example, has traditionally been made out of polysilicon, though many metals may be used instead and has better performance characteristics in certain areas. On the other hand, metals may present more challenges in the fabrication process.
One material that is now used in an attempt to optimize gate performance is referred to as a silicide. A silicide is basically an alloy of poly and a metal such as nickel, titanium, or cobalt. Silicides are sometimes used to create contacts, typically at the top of a gate electrode or a source or drain. They may, however, be used for the entire gate electrode. One way to fabricate such a gate electrode is to begin with a conventional poly gate, and then overlay the poly gate electrode with one or more metal layers after the remaining features have been masked with a dielectric or photoresist. The entire structure is then annealed, that is sufficiently heated for a sufficient length of time in order to combine all of the poly with the metal, down to the gate dielectric. A gate constructed in this or similar fashion is sometimes referred to as a fully-silicided (FUSI) gate because all of the poly material is consumed in the process.
These FUSI metal gates perform satisfactorily in many respects, but sometimes present a problem with attaining the proper work function, especially in PMOS devices. The work function basically describes the energy needed to move an electron in the solid atom from an initial Fermi energy level to vacuum level, that is, to outside the atom. Work function is a metric commonly applied to gate structures in transistors. An un-silicided poly gate, for example, commonly has a work function of approximately 5.0 eV. The work function in effect determines the threshold voltage at which the device will turn on or turn off. Silicided PMOS devices are frequently not able to obtain an acceptably high work function of, for example, 4.8 eV. This may result in a higher device threshold voltage, for example 0.2V, which may frustrate applications involving low-threshold-voltage logic.
Needed then is a way to achieve the advantages gained through the use of FUSI metal gates in semiconductor devices such as transistors while at the same time achieving a desirable work function that will permit exploitation of multi threshold voltage applications without the need for more (or more extensive) pocket or channel implants. The present invention provides just such a solution.